Pneumatic Instruments

Pneumatic transmitters
(courtesy of Foxboro)

Air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure transmitter designed to output a variable air pressure according to its calibration rather than a variable electric current. Such a transmitter would have to be supplied with a source of constant-pressure compressed air instead of an electric voltage, and the resulting output signal would be conveyed to the indicator via tubing instead of wires:

The indicator in this case would be a special pressure gauge, calibrated to read in units of process pressure although actuated by the pressure of clean compressed air from the transmitter instead of directly by process fluid. The most common range of air pressure for industrial pneumatic instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process measurement scale and an output pressure of 15 PSI represents the high end of the measurement scale. Applied to the previous example of a transmitter calibrated to a range of 0 to 250 PSI, a lack of process pressure would result in the transmitter outputting a 3 PSI air signal and full process pressure would result in an air signal of 15 PSI. The face of this special “receiver” gauge would be labeled from 0 to 250 PSI, while the actual mechanism would operate on the 3 to 15 PSI range output by the transmitter. As with the 4-20 mA loop, the end-user need not know how the information gets transmitted from the process to the indicator. The 3-15 PSI signal medium is once again transparent to the operator.

Typically, a 3 PSI pressure value represents 0% of scale, a 15 PSI pressure value represents 100% of scale, and any pressure value in between 3 and 15 PSI represents a commensurate percentage in between 0% and 100%. The following table shows the corresponding current and percentage values for each 25% increment between 0% and 100%. Every instrument technician tasked with maintaining 3-15 PSI pneumatic instruments commits these values to memory, because they are referenced so often:

Using the Foxboro model 13A pneumatic differential pressure transmitter as an example, the video below highlights the major design elements of pneumatic transmitters, including an overview of "maximum working pressure" versus "maximum measurement range" pressure.

The Foxboro model 13A pneumatic d/p cell transmitters measure differential pressure and transmit a proportional pneumatic output signal.

The information above is attributed to Tony Kuphaldt and is licensed under the Creative Commons Attribution 3.0.

Innovative Pressure Transmitter Automatically Selects Calibration Range

Foxboro S Series

Typical 2-wire intelligent transmitters require the operator to manually program and use a single calibration range suited to the specific application. Not anymore. A winner of a 2015 Flow Control Innovation Award has come up with a significant change in transmitter set-up.

Foxboro, through its patented “Foxcal” firmware allows the Foxboro models IDP10S, IGP10S, IAP10S transmitters to automatically select and use any of 11 preset calibration ranges (stored in firmware). These calibration ranges cover the full pressure range of the transmitter. Upon installation, the S Series transmitter automatically selects the appropriate calibration range based on application inputs; and, if application inputs change, automatically transitions to another, more appropriate calibration range – all while maintaining a reference accuracy of 0.05%.

It is the first pressure transmitter to incorporate not only multiple calibration curves, but the ability to automatically select and transfer between them in real time. With the new technology, users have a wide-range capability with high reference accuracy for all industries requiring precise differential, gauge, and absolute pressure measurement. Additional benefits are inventory reduction and simplified tech training. Because of their wide turndown range with such high reference accuracy, adopting one model of S Series transmitter eliminates the need to inventory, learn, and maintain multiple transmitter models that handle more limited ranges (e.g., 150 psi, 800 psi, 4000 psi).

The Foxboro IDP10S datasheet can be downloaded here.

Or, you can review it online below:

Foxboro IDP10S Intelligent Transmitter from Mead O'Brien, Inc.

For more information, contact:
Mead O’Brien
(800) 892-2769
sales@meadobrien.com
www.meadobrien.com